1. Reference to Related Patent
The present application is related to U.S. application Ser. No. 473,489, filed Feb. 2, 1990, by Paul V. Cooper, entitled "Melting Metal Particles," (hereinafter the "Melting Metal Particles Patent"), and now abandoned, which is a continuation-in-part of U.S. Pat. No. 4,898,367, application Ser. No. 222,934, filed Jul. 22, 1988, by Paul V. Cooper, entitled "Dispersing Gas into Molten Metal," (hereinafter the "Dispersing Gas Patent"), the disclosures of which are incorporated herein by reference.
2. Field of the Invention
The invention relates to melting metal particles and, more particularly, to techniques for rapidly melting scrap particles of light metals such as aluminum and to dispersing gas therein.
3. Description of the Prior Art
Light gauge, low density scrap metal particles such as chips, borings, and turnings are produced as a by-product of many metal processing operations. A significant amount of scrap metal also exists in the form of metal cans, particularly aluminum cans and used beverage containers. For convenience, all such scrap metal will be referred to herein as "scrap metal" and "particles." In order to recover the scrap metal for productive use, it is necessary to remelt it. Unfortunately, a number of problems are presented when scrap metal is attempted to be remelted. These problems are particularly acute in the case of light metal such as aluminum due to the tendency of the metal to oxidize when melted. The problems are worse for small particles of scrap metal than large ones, because (1) small particles have a relatively large surface-to-volume ratio and (2) small, lightweight particles tend to remain on the surface of a melting bath where they are oxidized while large, heavier particles sink rapidly beneath the surface without oxidizing.
Reverberatory furnaces have been used to melt scrap metal, but it has been necessary to use mechanical puddlers to achieve respectable recovery rates when small particles of scrap metal are being melted. Puddlers are expensive, bulky, mechanically complex, and are a source of iron contamination. Even with mechanical puddlers, melting of the scrap metal occurs slowly so that the metal tends to oxidize before it melts, resulting in recovery rates that are less than desirable. "Recovery rate" as used herein can be defined as follows: ##EQU1##
The situation is improved when induction furnaces are used. Strong inductive currents are set up in the molten metal which create a stirring action that rapidly submerges the scrap metal before additional oxide can form on the surface. Furthermore, the absence of high temperature combustion produces little or no oxide formation. The result is that recovery rates on the order of 97 percent can be attained. The chief drawback of the induction furnace melting technique is the high initial cost of the furnace and its relative small capacity with respect to a reverberatory furnace. The cost can be so great as to make the scrap recovery process uneconomical despite the high recovery rates available. A further drawback of the induction furnace melting technique is that it is a batch process, rather than a continuous process.
A different approach to the problem of recovering scrap metal is disclosed in U.S. Pat. No. 3,272,619 (hereafter the '619 patent), to V. D. Sweeney et al., the disclosure of which is incorporated herein by reference. In the '619 patent, molten metal is circulated from a reverberatory furnace, through an external crucible where a vortex is established, and back into the furnace. Melting of scrap metal does not occur in the furnace. Rather, the scrap metal is introduced into the vortex established in the external crucible. As the scrap metal swirls down in the vortex, the scrap metal particles eventually are melted. By appropriate control of such parameters as the temperature of the molten metal being circulated, the moisture content of the particles, and the rate at which the particles are fed into the crucible, recovery rates of about 90 percent can be attained.
Although the system described in the '619 patent has been reasonably effective, certain problems remain. The '619 patent states that the intensity of the vortex can be adjusted to produce desired submerging rates, but such adjustment has proven difficult to achieve in practice. The high surface tension of the molten metal in the crucible permits solid particles to remain on the surface of the vortex completely down into the return pipe to the furnace. The result is that solids and air can reach the furnace, with a consequent lowering of melting efficiency. In effect, the scrap metal being melted is exposed excessively to air such that undesired quantities of dross are formed. It is possible that oxide-covered metal drops (referred to hereafter as "agglomerations") can pass completely through the crucible and back into the furnace. An additional concern related to the device according to the '619 patent is the sensitivity of the crucible to flow variations. Because the crucible is most efficient with metal flowing near the top, a slight increase in flow rate can cause a spillover. Additionally, such a high operating level in the crucible can cause loss of through the crucible itself.
The apparatus disclosed in U.S. Pat. No. 4,747,583, issued May 31, 1988 to Gordon, et al. represents an improvement over the device ac to the '619 patent. In the '899 patent, metal particles are mixed with molten metal flowing in a vortex in a crucible by means of stationary blades that project radially outwardly from a vertically-oriented sleeve disposed within the crucible. The blades are arranged relative to the surface of the molten that particles deposited onto the surface of the molten are submerged substantially immediately after being into the flow of molten metal. This result is brought about by encountering the blades which cause the molten metal, with the particles entrained therewith, to be deflected downwardly.
In U.S. Pat. No. 4,598,899, issued Jul. 8, 1986 to Paul V. Cooper, melting of scrap metal particles is accomplished by disposing an auger in a bath of molten metal, rotating the auger so as to draw molten metal downwardly into the auger, and depositing metal particles onto the surface of the molten metal bath. By virtue of the action of the auger, the particles are drawn downwardly, through the auger, where they are forced into intimate contact with the molten metal and thereby are melted. Although the device disclosed in the '899 patent is very effective, certain concerns are not addressed. The auger disclosed in the '899 patent is a so-called shrouded auger, that is, it includes a plurality of radially extending blades, or flutes, that are surrounded by a hollow cylinder at their outermost ends. The relatively complex shape of the auger makes it relatively expensive and difficult to manufacture. The auger additionally is somewhat sensitive to the depth of molten metal in the bath, and the spaces defined by the blades and the surrounding hollow cylinder have the potential to become clogged with metal particles.
The device disclosed in the Melting Metal Particles Patent represents an improvement over the device according to the '899 patent. In the Melting Metal Particles Patent, a shaft-supported, rotatable impeller is immersed into a bath of molten metal and is rotated. Rotation of the impeller establishes a vortex-like flow. Metal particles are deposited onto the surface of the molten metal in the vicinity of the impeller. Due to the action of the vortex, the metal particles are submerged almost immediately.
The particular impeller used in the Melting Metal Particles Patent has proven very effective. The impeller is in the form of a rectangular prism having sharp-edged corners that provides an especially effective mixing action. The use of a shroud is not required. Due to the simplistic configuration of the impeller, it is inexpensive and reliable, while surprisingly being quite effective in operation.
Although the device disclosed in the Melting Metal Particles Patent is effective in quickly mixing the metal particles with the molten metal, certain concerns have not been addressed. One of these concerns relates to the strength of the vortex that can be established. The impeller in the Melting Meal Particles Patent must be operated relatively close to the surface of the bath in order to establish a strong vortex that will submerge the metal particles effectively.
Desirably, a technique would be available for rapidly mixing metal particles with molten metal that would be (1) inexpensive, (2) usable with a variety of containers (just not a crucible), (3) reliable, (4) long-lived, and (5) effective in its mixing action, particularly by being able to establish a strong vortex at a location relatively deep within a bath of molten metal. It also is desired that any mixer be able to be operated at the lowest possible speed while attaining good mixing results. It also is desired that any such device be configured so that it will be difficult or impossible to clog the device with metal particles.